Production of synthesis gas using convective reforming

ABSTRACT

A convective reformer device is provided which is useful for partially reforming a feed mixture of hydrocarbons and steam. The device includes an outer shell enclosure and a tubular inner core assembly and is specifically adapted to support heat exchange between a heating fluid, which may be an effluent from downstream, and the hydrocarbon-steam feed mixture. The convective reformer is used in a system and process for the steam reformation of hydrocarbons, in which the partially reformed effluent from the convective reformer is further reformed in a steam reforming furnace, or an auto-thermal reformer, or a steam reforming furnance followed by an auto-thermal reformer. The fully reformed effluent from the steam reformer, or auto-thermal reformer, is directed back to the convective reformer to supply the heat of reaction for the partial reformation of the feed mixture. This heating fluid may be directed through the tube portion of the convective reformer, or alternatively, through the shell portion, either of which may be filled with catalyst. The gas that emerges from the convective reformer, which has undergone heat exchange with the feed mixture may be used to preheat the feed mixture to the convective reformer.

This is a divisional of application Ser. No. 749,869 filed June 27, 1985now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is directed to an improved process and system for theproduction of hydrogen-rich gas by the steam reforming of hydrocarbonsby indirect heat exchange. The device used herein is similar to a shelland tube heat exchanger but the tubes or the shell are filled withreforming catalyst. In this invention, the device is called a"convective reformer". Additionally, this invention addresses the use ofthe "convective reformer" in unique low energy processing configurationsfor the manufacture of hydrogen-rich synthesis gas for the production ofammonia, methanol, hydrogen, oxo synthesis gas, reducing gas for thereduction of iron ore, and other processes, in a most thermallyefficient manner.

2. Description of the Prior Art

A number of processes have been described in which hot effluent streamsemanating from the process are used to provide convection heating to aportion of the process.

Wadsworth, U.S. Pat. No. 1,692,476 discloses a method and apparatus forcracking and rectifying pertroleum oils in which hot effluent is used topreheat the charge.

Wolf, U.S. Pat. No. 1,742,888 discloses a process for convertinghydrocarbons and mineral oils in which the hot effluent from a heatingfurnace is used to preheat entering oil.

Podbielniak, U.S. Pat. No. 2,123,799 discloses a process for the of heattreatment of hydrocarbon gases in which effluent from a conversionfurnace preheats an incoming gas charge.

Berg et al., U.S. Pat. No. 2,809,922 discloses a catalytic conversionprocess and apparatus having improved temperature control of thereaction and which makes extensive use of heat interchange betweendifferent portions of the apparatus.

Vorum, U.S. Pat. No. 3,278,452 discloses a method of producinghydrogen-containing gases in which steam for reforming and natural gasfor reacting are formed and heated in the convective portion of aprimary reformer before being passed to a secondary reformer.

Marshall, Jr., U.S. Pat. No. 3,367,882 discloses an ammonia synthesisgas process in which the hydrogen gas feed is preheated by convectionmeans from reformer heat before passing into the primary reformer. Wasteheat boilers make use of the hot effluents in the system to generateprocess steam.

Wirth et al., U.S. Pat. No. 3,661,767 discloses a fluid coking-steamcracking combination process wherein heavy and light fractions pass fromrespective sections of the coker vessel through the convection sectionof the steam cracker, before being returned to the coker or removed asproduct.

Dougherty, U.S Pat. No. 3,705,009 discloses heat recycling for ammoniapreparation in which process natural gas passes through a convectivepreheater portion of a primary reformer apparatus. This portion is alsoused to heat process air for the secondary reformer. Waste heat boilersare used to recover heat from the reformer effluents.

Bacsik, U.S. Pat. No. 4,321,130 discloses the preheating of feed gas inthe convection portion of a pyrolysis furnace tubular reactor beforepassing to a primary fractionator, and the use of fractionator effluentto preheat combustion air by employing a bottom pump around, top pumparound and/or quench water streams.

Crawford et al., U.S. Pat. No. 4,162,290 discloses a process forreforming hydrocarbons into hydrogen and ammonia, in which a convectivereformer is used in parallel with a primary reformer and sensible heatfrom a secondary reformer effluent is used as the heat source for theconvective reforming section.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a device that is useful tosteam reform hydrocarbons to produce a hydrogen-rich gas by indirectheat exchange with hot waste streams.

It is another object of this invention to provide a number of processingconfigurations whereby the thermodynamic efficiency of conventionalreforming processes is improved by using high temperature waste heatmost advantageously through the means of the above mentioned "convectivereformer " device.

It is a further object of this invention to provide a means for reducingthe size and fuel consumption of the primary reforming furnace in asteam reforming process.

These and additional objects which will be apparent from the descriptionthat follows are accomplished by the practice of the inventionsummarized below.

In brief, this invention in one of its aspects comprises an improvementin a reforming process for the manufacture of synthesis gas for theproduction of hydrogen, ammonia, methanol, oxo synthesis and iron orereduction gases. The invention also comprises in another aspect anapparatus (device) for the catalyzed steam reformation of a hydrocarbonfeed gas mixture prior to its further conversion in a conventionalreforming furnace or auto-thermal reformer. In this device, theendothermic heat of reaction for the steam reformation reaction issupplied by heat exchange with a hot reformed gas coming from an outsidereforming furnace or auto-thermal reformer, or even from a secondaryreformer in cases where primary and secondary reformers are used such asin the production of ammonia.

The system design and process of the present invention provideadvantages over the arrangement shown in U.S. Pat. No. 4,162,290. In thepatent, exchanger-reactor 28 is shown in parallel with reforming furnace13 in FIG. 1. The same is true of exchanger-reactor 74 and reforming 53in FIG. 2. In the present invention, the convective reformer whichprovides a means of heat exchange is in series, not parallel, with thereforming furnace. This in-series arrangement enables the use ofoperating conditions which are easier on the equipment and less likelyto lead to failure. For instance, with the present in-series systemdesign, only 15 to 25% of the methane in the feed material is reformedin the convective reformer. This lower degree of conversion is readilyachieved at relatively low operating temperatures of 1150° to 1250° F.generally. In contrast, the process of the patent, which typicallyinvolves reformation of 45% or more of the methane in the feed mixture,employs higher temperatures, for instance, as high as 1492° F. At thesemore severe conditions, specifically, higher tube wall temperatures, theservice life of the exchanger-reactor type device will be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 1A illustrate, in cross-section, a preferred convectivereformer device in accordance with this invention.

FIG. 2 is a schematic diagram showing how the convective reformer deviceof FIG. 1 may be used in a process for the production of ammonia whichalso employs primary and auto-thermal reformers.

FIG. 3 is a schematic diagram which shows how the convective reformerdevice may be used in another process for the production of ammonia,this one employing a powerformer device (such as the powerformerdescribed in U.S. Pat. No. 3,958,951, incorporated herein by reference).

FIG. 4 is a schematic diagram of a process for methanol production usingthe convective reformer device.

FIG. 5 is a schematic diagram showing how the convective reformer deviceis used in a process to produce hydrogen and oxo synthesis gases.

FIG. 6 is a more detailed schematic diagram of a process according tothe invention, using a convective reformer device as shown in FIG. 1 inthe production of a hydrogen-rich synthesis gas which, in turn, may beused in the production of ammonia as illustrated in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION AND DRAWINGS

The convective reforming device of this invention in its broadestaspects comprises:

(1) an outer shell enclosure comprising (i) two end portions, (ii) acylindrical main body portion, and (iii) separate inlet and outlet meansfor the conveyance of a heating fluid, each of which includes perforateddistributor plate means for delivering the heating fluid substantiallyuniformly to and from

(2) a core assembly within the outer shell enclosure, which comprises(i) separate inlet and outlet conduit means at opposite ends for theflow-through of the feed mixture of hydrocarbons and steam, the inletand outlet conduit means extending through outer shell enclosure (1) tothe outside of the convective reformer device, and (ii) a multiplicityof tubular conduits which are open to the path of feed mixture flow, theconduits being adapted to contain a particulate solid catalyst forcontacting with the feed mixture; wherein outer enclosure (1) and thecore assembly (2) are separated by a passageway for the heating fluidwhich is in open communication with the heating fluid inlet and outletmeans of outer shell enclosure (1), the passageway surrounding tubularconduits (ii) of core assembly (2).

The process for convective reformation of this invention in its broadestdescription comprises the following sequential series of steps, whichare conducted on a continuous basis:

(a) delivering a feed mixture of hydrocarbons and steam to a firstreformation zone which comprises convective reformation means;

(b) partially reforming the feed mixture in the first reformation zone;

(c) delivering the partially reformed effluent from the firstreformation zone to one or more additional reformation zones;

(d) further reforming the partially reformed effluent in the additionalzone or zones into a hydrogen-rich gas;

(e) directing at least a portion of the hydrogen-rich gas back to thefirst reformation zone;

(f) effecting heat-exchange between the hydrogen-rich gas and the feedmixture in the first reformation zone such that the heat of reaction forthe partial reformation reaction in this zone is thus supplied, and

(g) passing at least a portion of the heat-exchanged hydrogen-rich gasfrom step (f) to one or more additional heat-exchange zones, locatedupstream of the first reformation zone of (a), for use as a heatingfluid therein.

In step (g), the hydrogen-rich gas may be used to help generate freshamounts of high pressure process steam for the process, or to preheatthe feed mixture to step (a), or both.

Typically, in conventional steam reforming processes, hydrocarbons andsteam are contacted over a steam reforming catalyst under conditionsthat are conducive to the formation of a hydrogen-rich gas. In suchprocesses, gaseous hydrocarbons, such as natural gas, and vaporizableliquid hydrocarbons, such as liquidified petroleum gas and naphtha, areused as the feed material. The most common steam reforming catalyst incommercial procedures is nickel. In general, the nickel catalyst ismanufactured as nickel oxide on an inert support and, in turn, reducedto nickel in situ with hydrogen.

The steam reforming reaction is generally carried out using steam tocarbon ratios from 3:1 to 4.5:1, on a weight basis. Conventionally, afeed comprising the hydrocarbon and steam mixture is heated to atemperature at which the reforming reaction begins to occur. The feed isthen further heated in catalyst-filled tubes in the radiant section of areforming furnace. The degree of conversion of methane in the feed tohydrogen is a function of the operating temperature and pressure in thefurnace, with the conversion reaction being favored by the use of hightemperatures and low pressures. The specific temperatures at which thehydrocarbon feed is heated are dependent upon tube size and metallurgy.For instance, at elevated temperatures of about 870° C., pressures ofabout 24 atmospheres are used for those tube sizes normally employed incommercially available reforming furnaces. For lower operatingtemperatures of about 800° C., on the other hand, a pressure of about 32atmospheres is used for such devices.

Turning now to the drawings, in FIG. 1 it can be seen that theconvective reformer device according to this invention is capable offunctioning as a heat exchanger which contains catalyst-filled tubes.

Referring to FIG. 1, which illustrates a preferred embodiment,convective reformer device 2, in accordance with this invention,comprises outer shell 4, which can be made of carbon steel, and innercore assembly 6, which is of stainless steel. Outer shell 4 includeselliptical head portion 8, main body portion 10, flanged enclosures 12,inlet distribution annulus 14, outlet collection annulus 16, inlet port18 and outlet port 20. Outer shell 4 is lined with a layer of heatinsulation material 22, which is preferably made of blanket typeinsulation having a thickness of 4 inches.

Inner core assembly 6 comprises multiple steel tubes 24, which areparallel to one another and to the fluid flow through conduit 26. In thepreferred cases, from about 250 to about 1000 tubes are employed. Coreassembly 6 also contains disc-type baffles 28 and donut-type baffles 30,which are equally spaced from one another. Core assembly 6 is separatedfrom outer shell and insulation layer 22 by cavity 32, which surroundsmost of the core assembly, as also shown in FIG. lA.

Referring again to FIG. 1, convective reformer 2 also comprises inletdistribution annulus 14 and outlet collection annulus 16, containingperforated distributor plates 38 and 40, respectively. Distributorplates 38 and 40 serve to insure that the heating fluid flows into coreassembly 6 and around the tubes evenly, and that it emerges the tubearea uniformly to collect in duct 16. Baffle sets 28 and 30 function toincrease the velocity of fluid which enters through 14 and passes aroundand in contact with the tubes, thereby increasing the degree or rate ofheat transfer. Tubes 24 are sufficiently constricted in orifice size attheir lower ends so as to retain the catalyst while insuring the uniformflow of feed gas stream 36 to each of the tubes. Manways 42 and 44permit access to the inside of the convective reformer. Top manway 44 isuseful for the filling and emptying of catalyst in tubes 24. Flangedenclosures 12 are included to facilitate the hydrostatic testing of theshell enclosure.

In practice, hot effluent stream 34, from downstream in the process,enters convective reformer device 2, through inlet distribution annulus14 and perforated plate 38, travels through cavity 32, and thus aroundand in contact with core assembly 6, and leaves through outletcollection annulus 16 and perforated plate 40. Simultaneously, processfeed stream 36, which is being treated in accordance with the invention,enters inner core assembly 6 through conduit 26 and inlet port 18, andtravels through tubes 24, whereby it is heated in the presence of thecatalyst and thus partially reformed. Preferably, from about 15 to about25% of any methane present in the feed mixture is reformed in this step.The heat of reaction is supplied by exchange through the tube walls withhot effluent stream 34 on its path through cavity 32. Thus, streamserves as a heating fluid. The partially reformed gas stream then leavescore assembly 6 through outlet port 20.

Further details of preferred features for FIG. 1 of the inventionpracticed as described above are as follows:

Baffles: Five (5) disc-type, four (4) donut-type, equally spaced;

Tubes: Three hundred and twenty-six (326) in number, 3 inch outerdiameter by 14 B.W.G. seamless stainless steel, minimum wall thicknessof 0.095 inch, on a 3 3/4inch pitch strength welded into tube sheets;

Perforated Distributor Plates:

(a) Inlet Distribution

Annulus: Fifteen hundred (1500) 1/2-inch diameter equally spaced holesall around, arranged in six (6) rows;

(b) Outlet Collection

Annulus: Twelve hundred and fifty (1250) 1/2-inch diameter equallyspaced holes all around, arranged in five (5) rows.

It should be noted that the particular number of tubes and the shellsize of the convective reformer device is subject to variation toincrease or decrease their capacity, depending on productionrequirements.

The convective reformer device of this invention is intended to beoperated as the first in a series of two or more reforming stages. Thesecond stage may be a reforming furnace followed by still another, orthird, stage such as an auto-thermal reformer, as shown in FIGS. 2 and6. The second stage can alternatively be a powerformer device, which isin essence another convective reformer but of a different internaldesign, as shown in FIG. 3. The second stage may also be an auto-thermalreformer, as illustrated in FIGS. 4 and 5. Air, enriched air, ormolecular oxygen are used to adiabatically burn a portion of theeffluent gases from the convective reformer device and/or the reformingfurnaces and to supply heat for reforming the remainder of the unreactedmethane in the feed streams in these illustrated systems.

Referring to FIG. 6, a hydrocarbon feed, which may be natural gas, LPGor naphtha, is introduced to the system in conduit 50, and is preheatedto a temperatures of about 400° C. in preheater 52. The thermal energyfor preheater 52 is supplied by the convective section of reformingfurnace 72. The gaseous hydrocarbon feed is then passed from preheater52, through conduit 54, to reactor 56, which is of conventional design,where it it is pretreated in the usual manner to remove undesirableconstituents, such as sulphur compounds. The pretreated gaseous effluentflows from reactor 56 through conduit 58, where it is admixed withflowing process steam introduced through valve 60. The resulting mixtureof gaseous hydrocarbons and process steam is passed through conduit 58to preheater 62, where it is heated to a temperature of 968° F. beforebeing introduced through conduit 64 into convective reformer device 66.Preheater 62 is a conventional gas-to-gas heat exchanger.

Convective reformer 66 contains a tubular portion 68, which is filledwith a reforming catalyst, such as the catalyst described above. Theheat of reaction for the reforming reaction in convective reformer 66 issupplied solely by the gaseous effluent coming from auto-thermalreformer 78 through conduit 80. The hydrocarbon and steam mixture fed toconvective reformer 66 is heated and contacted with reforming catalystand it undergoes a partial reforming reaction. The partially reformedgas leaves convective reformer 66 at a temperature of about 1150° toabout 1250° F., through conduit 70 for introduction into tubular section74 of reforming furnace 72. Reforming furnace 72 can be of conventionaldesign and operated in the standard manner. The gaseous effluent fromfurnace 72, which is preferably at a temperature of about 1400° to about1500° F., most preferably 1460° F., flows through conduit 76 toauto-thermal reformer 78, where it is introduced together with processair.

The process air is fed to the system through compressor 82, then throughconduit 84 to preheaters 86 and 88, respectively, before enteringauto-thermal reformer 78 to take part in the reaction. The heating ofthe process air stream in preheaters 86 and 88 is brought about by theuse of process waste heat, as shown. Preferably, the process air has atemperature of about 1500° F. before its introduction into auto-thermalreformer 78.

The gaseous effluent emitted from auto-thermal reformer 78, which is afully reformed gas, is preferably kept at a temperature of 1500° to1870° F. The fully reformed effluent is directed through conduit 80 toconvective reformer 66, where it serves as a heat exchange medium toprovide the heat of reaction for the reforming reaction taking placethere. This fully reformed gaseous effluent enters convective reformer66 at a temperature of about 1840° F., typically, but it emerges at alower temperature of about 1360° F. as a result of the heat exchangephenomenon that takes place. The emerging gas is conducted throughconduit 90 for further heat recovery using conventional techniques asshown in FIG. 2.

If desired, a portion of hydrocarbon feed stream 50, shown as stream 92,may be directed through the convective section of reformer furnace 72,where it is preheated, and then to the burners of furnace 72 where itserves as fuel gas for the heating of the process gas in tubular section74.

The gaseous effluent from auto-thermal reformer 78 in FIG. 6 iscomprised chiefly of a mixture of nitrogen, hydrogen, carbon monoxide,carbon dioxide and a small amount of methane. As shown in FIG. 2, afterthe shifting of carbon monoxide to hydrogen and carbon dioxide, thesubsequent removal of carbon dioxide and methanation of any carbonoxides, a mixture of nitrogen and hydrogen is then synthesized intoammonia.

In FIG. 3, the process steps are essentially the same as in FIG. 6, butthe reforming furnace has been replaced by a powerformer. In thisembodiment, the hydrocarbon feed, after being preheated, is partiallyreformed in a convective reformer, using the effluent from anauto-thermal reformer as the heat source, as shown. The reformingcontinues in the powerformer, where pressurized flue gas is used as theheat source, as can be seen. The pressurized hot flue gas emerging fromthe convective reformer is expanded through an expander turbine, whichalso drives the combustion air compressor for the burner. The finalstage of reforming is achieved in the auto-thermal reformer. Theprocessing sequence after the completion of reforming, to produceammonia synthesis gas, is the same as shown in FIG. 2.

The application of the convective reformer in series with anauto-thermal reformer only is shown in FIGS. 4 and 5. In FIG. 4, thesynthesis gas is tailored to produce methanol as the end product. InFIG. 5, after the carbon monoxide (CO) shift and the carbon dioxide(C0₂) removal, the effluent may be methanated to produce either hydrogenor, alternatively, ammonia synthesis gas.

In FIGS. 4 and 5, a feed mixture of gaseous hydrocarbon and steam ispartially reformed in a convective reformer. Using the hot effluentgases from the auto-thermal reformer as a heat source, a portion of thegaseous effluent from the convective reformer is combustion burned withoxygen in the auto-thermal reformer, to provide the heat of reformingfor the remainder of the gas stream. The product of the combustion, thatis the carbon dioxide formed, serves to increase the ratio of carbon tohydrogen in the reformed gas such that the gas is richer in carbon andthe hydrogen/carbon oxides ratio is nearly stoichiometric for themethanol reaction. In the normal case, when reforming natural gas thesynthesis gas which is produced is rich in hydrogen and, consequently,the hydrogen must be purged from the synthesis section unless anexternal source of carbon dioxide is available. This carbon dioxide ismixed with the hydrogen-rich synthesis gas produced to get the desiredstoichiometric carbon oxides to hydrogen ratio. With the presentinvention, however, the desired ratio is achieved in a morecost-effective and convenient manner.

Referring again to FIG. 5, the synthesis gas from the auto-thermalreformer, which consists of hydrogen, carbon monoxide and carbondioxide, is shifted to produce hydrogen. After the shifting, removal ofthe carbon dioxide and methanation, a relatively pure hydrogen gas isobtained. To produce ammonia, the methanation procedure is not requiredand can be omitted because the carbon oxides can be removed using aliquid nitrogen wash. The nitrogen wash removes the impurities and,concomitantly, supplies the quantity of nitrogen needed for the ammoniasynthesis.

In both FIGS. 4 and 5, the effluent from the auto-thermal reformer,which is redirected back to the convective reformer to serve as a heatexchange medium and to heat the incoming gas, is used further, after itemerges from the convective reformer, to preheat fresh feed prior tointroduction of the latter into the convective reformer and also togenerate steam for the process, as shown.

A benefit provided by the practice of this invention is that the naturalgas comsumption for the production of ammonia in particular is decreasedin the order of 6MM BTU/MT.

Other variations of the invention are possible. For instance, theconvective reformer device can be modified by placing the catalyst on asupport within the "shell" portion (the cavity identified as item 32 inFIGS. 1 and lA), and passing the feed mixture of hydrocarbons and steam(item 36, FIG. 1) through it, while simultaneously conveying the processeffluent that serves as the heating fluid (item 34 in FIG. 1) throughthe tubular portion (items 24, FIG. 1) of the inner core assembly. Theflows can be countercurrent as shown in FIGS. 2-6 or co-current as shownin FIG. 1. Still other modifications will occur to those skilled in theart in view of the above description without departing from the scope ofthe invention defined in the appended claims.

We claim:
 1. A process for the convective reformation of a mixture ofhydrocarbons and steam into a hydrogen-rich gas, comprising the stepsof(a) delivering substantially all of a feed mixture of hydrocarbons andsteam to a first reformation zohe which comprises convective reformationmeans; (b) partially reforming the feed mixture in the first reformationzone at a temperature sufficient to reform from about 15% to about 25%of the hydrocarbon in the feed mixture; (c) delivering the partiallyreformed effluent from the first reformaton zone to one or moreadditional reformation zones; (d) completely reforming the partiallyreformed effluent into a hydrogen-rich gas in the additional reformationzones; (e) directing at least a portion of the hydrogen-rich gas fromstep (d) back to the first reformation zone of step (a); (f) effectingheat-exchange between the hydrogen-rich gas from step (e) and the feedmixture of hydrocarbons and steam in the first reformation zone, thehydrogen-rich gas being initially at a higher temperature than the feedmixture, such that the heat of reaction for the partial reformation isthus supplied from this exchange; and (g) passing at least a portion ofthe heat-exchanged hydrogen-rich gas from step (f) to one or moreadditional heat-exchange zones located upstream of the first reformationzone of (a) for use as a heating fluid therein.
 2. A process accordingto claim 1, in which in step (g) the hydrogen-rich gas is used as aheat-exchange medium for the generation of high pressure process steamfor the process.
 3. A process according to claim 1, in which in step (g)the hydrogen-rich gas is used as a heat-exchange medium for thepreheating of the feed mixture to step (a).
 4. A process according toclaim 1, in which the partial reformation reaction in step (a) isconducted at a temperature of about 1150° to about 1250° F.
 5. A processaccording to claim 1, in which the feed mixture is characterized by asteam to carbon ratio from 3:1 to 4.5:1.